Among the heretofore known antennas, a half-wave dipole antenna is known as the antenna having the lowest directionality, or nondirectional in a way. Such half-wave dipole antenna is the antenna to be formed by arranging two antenna elements of linear shape in one straight line. And then it becomes an antenna pattern having a gain of doughnut shape in a direction orthogonal to such antenna element.
Moreover, as an antenna similar to the half-wave dipole antenna, a quarter-wave monopole antenna is known as well, which is formed by an arrangement using only either piece of the two antenna elements of the dipole on an electrically conductive board (a bottom board). According to such quarter-wave monopole antenna, it is possible to obtain a mirror image of the antenna element at a position symmetrical to the electrically conductive board, which has the length of one quarter wavelength and is arranged on the electrically conductive board. And then in a case where the electrically conductive board is assumed to be broadened infinitely, it is possible to obtain characteristics completely similar to that of the half-wave dipole antenna due to the quarter-wave monopole antenna and the mirror image thereof.
Such dipole antenna or monopole antenna is widely used conventionally as a nondirectional antenna. For example, the monopole antenna is used as an antenna on a roof of a vehicle, and as an antenna for a portable telephone. Moreover, as a type of monopole antenna to be used in actual circumstances, the one that is widely used, for example, has a structure wherein a central conductor of a coaxial line is used as an antenna element, and an outer conductor is connected to a bottom board.
While, as a radar device to be mounted on a vehicle and for detecting an obstacle or the like in a travelling direction thereof, one type is heretofore known, wherein a plurality of antennas are arrayed, and then an azimuth of the obstacle or the like is measured. For example, according to an antenna for radar device 900 as shown in FIG. 10, which is disclosed in patent document 1, there is disclosed an antenna to detect an azimuth of an obstacle by using an array antenna, which is formed by arraying a plurality of antenna units 902 on a bottom board 903, wherein an antenna element 901 is formed in a helical manner individually.
[Patent Document 1] Japanese Patent Application Publication No. 2006-258762